Wave energy commercialization lags well behind wind energy despite the fact that water is approximately 800 times denser than air, and waves travel over thousands of miles for days and even weeks after the wind which originally produced them has subsided. Waves, therefore, efficiently store wind energy at much higher energy densities, typically averaging up to 50 to 100 kw/m of wave front in many northern latitudes.
Hundreds of uniquely different ocean wave energy converters (OWECs) have been proposed over the last century and are described in the patent and commercial literature. Less than a dozen OWEC designs are currently ocean deployed as “commercial proto-types.” Virtually all of these initial designs suffer from high cost per average unit of energy capture. This is primarily due to the use of heavy steel construction necessary for severe sea-state survivability combined with (and in part causing) low wave energy capture efficiency. Only about 10% of currently proposed OWEC designs are deployed subsurface and/or can be submerged to greater depths where severe sea-state problems are substantially reduced. Most subsurface OWECs are, unfortunately, designed for near shore sea bed deployment. Ocean waves lose substantial energy as they approach shore (due to breaking or reflected waves and bottom hydrodynamic friction effects). Near shore submerged sea bed located OWECs must be deployed at greater depths relative to average wave trough depths due to severe sea-state considerations to avoid breaking wave turbulence, and depth can not be adjusted for the large tidal depth variations found at the higher latitudes where average annual wave heights are greatest. Wave induced subsurface hydrostatic pressure oscillations diminish more rapidly in shallow water as the depth below waves or swell troughs decreases.
Only a few prior art substantially or fully submerged (“subsurface”) devices use gas filled variable volume or deformable containers like the present invention, producing container expansion or contraction in response to overhead swell and trough induced static pressure changes. Only one of the prior art subsurface OWECs attempts to capture both hydrostatic (heave) and hydrokinetic wave energy (surge or pitch), which represents half of all wave energy, per the present invention. None of the prior art gas filled deformable containers use surface direct or low pressure indirect venting or breathing of container gas to increase container deformation or stroke and, therefore, wave energy capture. None of these prior art subsurface OWECs enhance or supplement energy capture with overhead floating bodies. All of the prior subsurface deformable container OWECs suffer from high mass (and therefore cost) and low energy capture efficiency (even more cost) usually due to either near shore and/or sea bed deployment and high moving mass. None of these prior art subsurface devices have the tidal and sea-state depth adjustability of the present invention needed for enhanced energy capture efficiency and severe sea-state survivability. None of the prior art devices have the low moving mass (allowing both short wave length and long swell energy capture) and the large deformation stroke (relative to wave height) needed for high capture efficiency like the present invention.
At least two prior art devices use two variable volume gas filled containers, working in tandem, to drive a hydraulic turbine or motor. Gardner (U.S. Pat. No. 5,909,060) describes two sea bed deployed gas filled submerged inverted cup shaped open bottom containers laterally spaced at the expected average wavelength. The inverted cups are rigidly attached to each other at the tops by a duct. The cups rise and fall as overhead waves create static pressure differences, alternately increasing and decreasing the gas volume and hence buoyancy in each. The rise of one container and concurrent fall of the other (called an “Archemedes Wave Swing”) is converted into hydraulic work by pumps driven by said swing.
Similarly, Van Den Berg (WO/1997/037123 and FIG. 1) uses two sea bed deployed submerged average wavelength spaced interconnected pistons, sealed to underlying gas filled cylinders by diaphragms. Submerged gas filled accumulators connected to each cylinder allow greater piston travel and hence work. The reciprocating pistons respond to overhead wave induced hydrostatic pressure differences producing pressurized hydraulic fluid flow for hydraulic turbines or motors.
The twin vessel Archemedes Wave Swing (“AWS”) of Gardner (U.S. Pat. No. 5,909,060) later evolved into a single open bottomed vessel (FIG. 2) and then more recently Gardner's licensee, AWS Ocean Energy has disclosed an enclosed gas filled circular section vessel (an inverted rigid massive steel cup sliding over a second upright steel cup) under partial vacuum rather than the surface venting or breathing of the present invention to reduce stroke limiting gas compression (FIG. 3). Partial vacuum is maintained via an undisclosed proprietary “flexible rolling membrane seal” between the two concentric cups. Power is produced by a linear generator (FIG. 2 shown) or hydraulic pump driven by the rigid inverted moving upper cup. An elaborate external frame with rails and rollers, subject to fouling from ocean debris, is required to maintain concentricity and preserve the fragile membrane. No attempt is made to capture any wave kinetic energy (surge) which represents 50% of total wave energy in deep water (and even more in shallow water).
FIG. 4 (Burns U.S. 2008/0019847A1) shows a submerged sea bed mounted gas filled rigid cylindrical container with a rigid circular disc top connected by a small diaphragm seal. The disc top goes up and down in a very short stroke in response to overhead wave induced static pressure changes and drives a hydraulic pump via stroke reducing, force increasing actuation levers. Bums recognizes the stroke and efficiency limitations of using wave induced hydrostatic pressure variations to compress a gas in a submerged container and attempts to overcome same by arranging multiple gas interconnected containers perpendicular to oncoming wave fronts. North (U.S. Pat. No. 6,700,217) describes a similar device. Both are sea bed and near shore mounted and neither is surface vented like the present invention to increase stroke and, therefore, efficiency or makes any attempt to capture wave kinetic energy (surge). North, U.S. Pat. No. 6,700,217 describes a container and small diaphragm seal very similar to Burns and also without gas venting.
FIG. 5 (Meyerand U.S. Pat. No. 4,630,440) uses a pressurized gas filled device which expands and contracts an unreinforced bladder within a fixed volume sea bed deployed rigid container in response to overhead wave induced static pressure changes. Bladder expansion and contraction within the container displaces sea water through a container opening driving a hydraulic turbine as sea water enters and exits the container. Expansion and contraction of the submerged bladder is enhanced via an above surface (shore mounted) diaphragm or bellows. High gas pressure is required to reinflate the submerged bladder against hydrostatic pressure severely reducing submerged bladder deformation.
FIGS. 6a and 6b show Bellamy's Circular Coventry Clam (1985) which locates 12 semi-submerged air bags around a common circular air duct. Air is displaced from some bags into others by waves, the displaced air passing through Wells air turbines. Capture efficiency is highly dependent on wave length and period and only those bags facing oncoming waves have any kinetic (surge) energy capture effectiveness.